The major components of a liquid crystal display (LCD) panel include a polarizer and a backlight module. The backlight module is provided for emitting light required by screen, and thus adjusting the brightness of a screen of an LCD panel is the same as adjusting the brightness of the light emitted by the backlight module. The brightness of the backlight module is controlled by current. The larger the current, the higher is the brightness. Referring to FIG. 1 for a circuit for adjusting the current of a backlight module, the circuit comprises a light adjusting signal source 1, a pulse controller 2, a switch 5, a power source 4, a transformer 6, a lamp set 7 and a sampling unit 3 for collecting and sending a feedback signal from the lamp set 7 to the pulse controller 2. The switch 5 controls the power source 4 to be supplied to a secondary side of the transformer 6, wherein the switch 5 includes a primary switch 51 and a secondary switch 52, and the duty cycles of both switches 51 and 52 are alternated, and a working voltage is generated at a secondary side of the transformer 6, such that the lamp set 7 is turned on by a voltage difference to emit light. The key of controlling the working voltage resides in that the pulse controller 2 provides a duty cycle signal for the operation of the switch 5, and the pulse controller 2 primarily bases on a voltage reference signal and a frequency reference signal, and the voltage reference signal produced by combining the feedback signal and a light adjusting signal that produces the light adjusting signal source 1. Referring to FIG. 1 for a circuit block diagram of a conventional amplitude modulation adjusting circuit, the frequency reference signal is generated primarily by connecting a capacitor to the pulse controller 2. Since the capacitance of the capacitor is constant, the charge/discharge time is constant, and the frequency reference signal as shown in FIG. 1 has a constant frequency. The voltage reference signal can be adjusted to change the duty cycle of the switch 5 to achieve the light adjusting effect, but the change of duty cycle of the switch 5 also gives a longer dead time between the duty cycles of the primary switch 51 and the secondary switch 52. As a result, zero voltage switching cannot be achieved, and losses occur. In another conventional frequency modulation adjusting circuit, the voltage is maintained constant and the operating frequency of the switch 5 is changed to achieve the light adjusting effect, but brightness of the lamp set 7 will be too low and the life expectancy of the lamp set 7 will be reduced, if the power frequency of the lamp set 7 is either higher or lower than the operating frequency of the lamp set 7. Furthermore, frequency interference (an interference with other frequencies of the panel) is produced at a high-frequency operation. As a result, the range of frequency change of the frequency modulation adjusting circuit is limited and the control range is limited, and a general user may feel that adjusting capacity is not as good, and a frequency interference (or a waveform with funny interferences will appear on the screen) may be produced easily, and the aforementioned two control method still require further improvements.